[UX] unable to reference more than one item in autocomplete using a comma

[jenlampton]

For other types of reference autocomplete fields, more than one term can be referenced by typing a comma, and then you can search for another. I think maybe the comma in this autocomplete field does not properly re-start the search for a second term. I can see the icon change as though it's doing a search, but no matches are returned even when I'm typing an exact match.

[jenlampton]

I've found some issues with the autocomplete callback, and was able to fix them by making it work more like the core taxonomy term reference autocomplete. PR on the way.